1. Field of the Invention
This invention relates to a process for forming slots in a semiconductor wafer and, more particulary, relates to a process for forming slots having sidewalls with a desired aspect ratio in regions near the surface.
2. Discussion of Background and Prior Art
The use of slots or trenches is becoming more common in isolating individual devices within an integrated circuit. For certain applications such slots or trenches will be replacing the traditional approaches of pn junction isolation and oxide isolation. See, e.g., D. N. K. Wang et al, "Reactive-Ion Etching Eases Restrictions on Materials and Feature Sizes", Electronics, Nov. 3, 1983, pp. 157,159. In addition to the use of slots in silicon wafers for isolating individual devices, such slots may also be used as active elements. For example, a slot may be filled with appropriate materials to serve as a capacitor, see e.g., K. Minegishi et al, "A Submicron CMOS Megabit Level Dynamic RAM Technology Using a Doped Face Trench Capacitor Cell", Proceedings IEDM 1983, p. 319. Whether a slot is fabricated for the purpose of isolating devices or for the purpose of being processed to serve as an active element, it is required that the slot formation process not interfere with the formation of other features of the integrated circuit and that the finished slot structure not have a deleterious impact on other structural features or on the electrical operation of the completed integrated circuit.
The traditional technique for fabricating slots, the so-called cut-and-fill technique, has generally not been widely implemented in the semiconductor industry because it has utilized chemical etches which anisotropically etch silicon along specified crystallographic planes. Except for special crystallographic orientations the shape of the etched regions tends to be a V with a high ratio of lateral spacing to depth. Also, while it has been possible to fabricate long narrow grooves in a wafer of precisely specified orientation with careful masking and etching, it has not been possible by chemical means to produce arbitrary shapes or small holes. The direction of etching follows the orientation of the crystal axes so the arbitrary shapes are not possible. See, e.g., D. L. Kendall, "Vertical Etching of Silicon At Very High Aspect Ratios", Annual Review of Material Science, 1979, v. 9, pp. 373-403.
The increasing use of slots or trenches as discussed above is due principally to the availability of physical etching equipment such as reactive ion etch equipment. Since the ion beams or plasma action may be directed, etching of arbitrary shapes may be accomplished. Chemically reactive species may also be used in the beams so that the benefits and effects of both physical and chemical etching may be obtained. And, there has been a proliferation of techniques such as selective etching and anisotropic etching using such equipment. See, e.g., H. W. Lehmann et al, "Dry Etching For Pattern Transfer", J. Vac. Science & Technology, v.17, No. 5, September October 1980, pp. 1177; L. M. Ephrath, "Reactive Ion Etching for VLSI", IEEE Transactions on Electron Devices, V. ED-28, No. 11, Nov. 1981, p. 1315; and D. N. K. Wang et al, "Reactive-ion etching eases restrictions on materials and feature sizes", Electronics, Nov. 3, 1983, p. 157. In practice, this equipment may be used to etch slots which are then filled with a suitable filler material, the excess filler material is then removed and the substrate and exposed filler material buried under overlying areas including metallization lines. The slot thus becomes one feature of a layered structure. It is necessary, therefore, that the slots not adversely affect the electrical properties of the surrounding regions or the physical integrity of these regions.
In the formation of slots it is generally desired that the walls have a high aspect ratio or be vertical since high aspect ratios permit deep slots to be formed without unduly great lateral dimensions. This increases the density of integrated circuits. High density integrated circuits would not be possible if lateral dimensions increased proportionally to depth. For example, for bipolar memories a slot may only need to be 5 .mu.m deep but for linear bipolar devices a slot may need to be as deep as 70 .mu.m. In the latter case the great depth would not permit high density devices to be fabricated if the lateral spacing was proportional to depth. In practice, after the slots are formed they are filled with material such as polycrystalline silicon, silicon dioxide, polyimide, or like materials. The slots are filled so that after excess filler material is removed the slots present a planar surface on which overlying layers such as conductive metal lines may be applied. If the walls of the slots have severe undulations or overhangs which shadow certain interior regions of the slot, then the slot can be imperfectly filled and voids formed in these shadowed regions. Or if they have bottle shapes there may be centrally disposed voids which are formed as the slots are filled by isotropic growth processes. If voids occur they can become exposed on the surface after removal of excess filler material. If the voids remain exposed during subsequent processing, the irregular topography would be a poor foundation for overlying layers, and particularly for overlying metal layers. The metal layers would be subject to breaking, as shown in the Prior Art of FIG. 1f, thereby lowering yield on a finished wafer. It is therefore highly desirable that slots be formed which do not have concavities, severe undulations or overhangs near the surface that would shadow the source of filling material or otherwise produce incomplete filling and allow voids to be generated which may subsequently be exposed on the surface.
Throughout the remainder of this specification the slot formation process of the present invention will be described in the context of silicon processing, the prevalent type of semiconductor material processing. The process applies as well, however, to other semiconductor materials such as germanium or III-V compounds such as gallium arsenide, indium phosphide, and like materials. The problem of irregular topography due to voids in the upper regions would need a solution in each of these processing regimes.
It is therefore an object of the present invention to provide a process for producing slots having a desired aspect ratio and near vertical orientation and without concavities or overhangs near the surface of the slot.
It is another object of the present invention to provide a process for producing slots which do not have a bottle shape, at least adjacent critical upper regions of the slot.
It is another object of the present invention to provide a process which coats the upper regions of a slot being formed to prevent those regions from experiencing unwanted etching as the etching process is continued to form the bottom regions of the slot.